1. Field of the Invention
This invention relates to air dispensing arrangements within gas turbine engines, and particularly to a new and improved air dispensing arrangement which permits higher and lower energy pressurized air to be selectively dispensed to engine components where that level of energy air will be most effective.
2. Description of the Prior Art
The pressurized air supplied by a compressor within a gas turbine engine has numerous uses. In one use, the pressurized air is introduced into the combustor of the engine wherein it is mixed with fuel and ignited. The resulting hot gases flow across a turbine, causing the turbine rotor to rotate. Work is extracted from the turbine and used, for example, to rotate a compressor, a fan, or a propeller.
The pressurized air supplied by the compressor is also used to cool engine components, such as the combustor walls and the turbine, both of which are heated by the hot combustor gases. The maximum temperature to which components, such as the cumbustor walls and the turbine, can be subjected is often an operating limitation of the engine. Therefore, if these components can be cooled more effectively, the combustor gases can be burned at a higher temperature without exceeding component temperature limitations. The ability to burn combustor gases hotter results in an increased range of engine performance.
With respect to the cooling function of the pressurized air, prior art air dispensing arrangements present certain limitations. More specifically, as the pressurized air exits the compressor, it passes through a duct, such as a diffuser. Because air has viscosity, the pressurized air flowing close to the walls of the duct is slowed. At the very surface of the duct walls, the air particles are slowed to a relative velocity of near zero. At greater distances from the duct wall surface, the air particles experience progressively smaller velocity retardation until at some point, the local velocity reaches the full value of airflow nearer the center of the duct interior. The layer of air flowing close to the duct wall surface which displays velocity retardation is called the "boundary layer".
The kinetic energy of the air is defined by the formula 1/2mv.sup.2, where "m " represents the mass of the air and "v"represents the velocity of the air. Due to decreased velocity, the energy of the air flowing within the boundary layer is lower than it is outside of the boundary layer. Thus, the pressurized air flowing adjacent the walls of the above-mentioned duct includes lower energy air while the pressurized air flowing nearer the center of the interior of the duct includes primarily higher energy air.
Most prior air dispensing arrangements within gas turbine engines are so disposed that the higher energy pressurized air exiting from near the center region of the duct interior is channeled through the combustor dome into the combustor to be mixed with fuel and burned therein. However, the air used to cool the combustor walls and turbine or other components, comprises the remainder of the air exiting the duct, namely, the lower energy pressurized air flowing from adjacent duct walls. The lower energy air is typically dispensed by conduits to flow along the outside of the combustor walls and is finally directed onto the turbine. Such lower velocity, lower energy air is able to accept less heat from hot engine components and is thus less effective for cooling the combustor walls and turbine than would be the higher velocity, higher energy air which, in prior art arrangements, is used primarily for combustion.
Additionally, many combustors employ dilution holes in the walls thereof to inject relatively large amounts of air into the combustor to dilute the hot gases therein. The air for such dilution is, in most prior art air dispensing arrangements, extracted from the lower energy air flowing along the outside of the combustor walls. Such extraction reduces even further the amount, and thus the effectiveness, of the lower energy air to cool the combustor walls and turbine.
In view of the above problems, it is therefore an object of the present invention to increase engine performance by providing an air dispensing arrangement having an improved capability for cooling engine components.
Another object of the present invention is to provide an air dispensing arrangement which can supply dilution air to a combustor without significantly affecting the cooling of engine components.